1. Field of the Invention
The present invention relates to the field of particle or ionizing radiation detectors, and in particular detectors of neutrons, γ- or X-rays.
2. Discussion of the Related Art
FIG. 1 schematically shows the conventional structure of a cell 2 sensitive to an ionizing radiation, using the same detection principle as the present invention. This cell comprises a conductive tube 4 filled with a gas mixture, sealed at its ends by isolating plugs 6. A conductive wire 8, the ends of which tightly cross plugs 6, is maintained tensed at the center of tube 4 by a spring 10 located within the tube. A positive electric voltage applied to wire 8 by means of a measurement circuit 12 enables defining within the tube an electric field which is favorable to the drifting and to the amplification of electrons generated at the passing of the ionizing radiation, which enters the tube in a direction approximately orthogonal to the axis of the tube. A resistive wire is used in a case where a position measurement along the tube is desired to be performed by charge division. The measurement circuit then comprises read electronics enabling measurement of the charge signal amplitude at each end of the wire. Another so-called “counting” operating mode uses electronics based on the comparison, with respect to a reference voltage, of the signal measured at a single end of the wire. The gas mixture contained in the tube is provided to be ionized by the particles which are desired to be detected, either directly, or after conversion into ionizing particles. For example, a mixture of CF4 and He3 in which He3 plays the role of a converter, and CF4 that of a stopping gas of the two ionizing particles (proton and triton) emitted after capture of a neutron by an He3 atom, is used in the case of neutron detection
The dimensions of tube 4 and the pressure at which the gas mixture is confined are very variable. As an example, tube 4 may have a width of approximately one meter, a diameter of approximately 8 mm and a thickness of approximately 0.2 mm, and the gas mixture may be confined in the tube at a pressure of approximately 15 bars. The forming of such a cell, which implies a perfectly tight welding of plugs 6 under a high pressure, after positioning of the wire, is particularly expensive. It is possible to provide individual filling means for each cell, but this creates an undesirable additional mechanical bulk.
Distance δ existing between the internal wall of tube 4 and spring 10 conditions the maximum electric voltage or breakdown voltage that can be applied between the electrodes and the tube. The larger the diameter of spring 10 with respect to the diameter of tube 4, the lower the breakdown voltage, at which electric arcs form between the spring and the tube wall. Further, the uniformity of the cell response is affected by the inaccuracy of the wire centering inside of the tube, and such a wire centering is difficult to perform by means of spring 10. In practice, the presence of spring 10 in the tube and the difficulty of the centering of wire 8 by means of spring 10 limit the maximum amplification gain with which the detector can operate, which has direct consequences upon the detector performances (energy and position resolution).
An ionizing radiation detector is conventionally formed of several cells 2, the tubes of which are juxtaposed and form a sensitive surface. The operation of a cell depends on the quality and on the pressure of the gas mixture that it contains. Now, it is difficult to form several sensitive cells comprising a same gas mixture with a long-term stability and identical for all cells. As a result, no sensitive cell really has an operation identical to the others.
The assembly of several cells requires an accurate mechanism. Further, when several sensitive cells must be used together with a minimum space between the tubes, it is difficult to ensure the continuity of the electromagnetic shielding between the tube envelope and measurement circuit 12 without extending beyond the external diameter of the tube, which results in creating dead spaces between cells, whereby a loss of sensitivity of the assembly. This constraint, and those imposed by inner spring 10, limit the minimum diameter of the tubes to approximately 7-8 mm. Further still, a sensitive cell may wear out and need changing, for example, if the gas mixture that it contains has been altered under the influence of the received radiation. Especially, it is known that a gas mixture of butane and argon contained in the sensitive cells used for the X-ray detection may form polymers around the wires under the effect of the radiation and alter the operation of the sensitive cell. The replacing of a cell is expensive.